This disclosure concerns ion storage materials for batteries of disposable (primary) or rechargeable (secondary) type, battery electrodes, battery cells, battery packs, and methods of using said batteries or battery packs so as to allow improved functionality as described herein. For many battery applications, it is desirable to know the state of charge of the battery while it is in use. The state of charge (SOC) is defined as a percentage of the capacity that the battery exhibits between a lower voltage limit at which the battery is fully discharged at equilibrium, and an upper voltage limit at which the battery is fully charged at equilibrium. Thus a 0% SOC corresponds to the fully discharged state and 100% SOC corresponds to the fully charged state. A battery, or string of batteries forming a battery pack, may be used over a limited range of SOC or over a wide range including the entire ion storage capacity available from the battery.
SOC monitoring is desirable or necessary in many battery applications, including portable electronics products such as wireless communications devices and laptop computers, power tools, electric vehicles (including hybrid, plug-in hybrid, and all-electric vehicles), backup power systems, energy storage for power generation devices such as solar or wind collectors or fuel cells or conventional fuel-burning power sources, and the like. In hybrid electric vehicles (HEVs) as one example, it is desirable to monitor the state of charge of the battery since operation does not typically use the whole range of SOC. For example, the SOC range used may be 10-90%, or 20-80%, or 30-70%, or 40-60% of SOC. Since the next pulse applied to the battery may be charge or discharge, most HEV packs are maintained at mid-SOC (40-60% SOC) where both the discharge and charge power capability are relatively high. It is very common to hold the battery at about 50% SOC since a battery operated in this manner can capture braking energy and, therefore, charge the battery further, or discharge energy in an acceleration step and, therefore, discharge the battery. Therefore, by sitting at a state of charge that is intermediate to a fully charged or discharged battery, the battery is able to meet the power requirements either for charge or for discharge at all times.
Another reason to monitor SOC accurately is to improve the life and/or safety of the battery. Some battery chemistries become unsafe at too high a charge voltage, and many chemistries degrade faster at very high or very low SOC. An accurate SOC estimate is therefore useful for improving system safety and/or long life.
The range over which a battery desirably operates is referred to as the state-of-charge swing. In addition to maintaining responsiveness to either a charge or discharge event, keeping the battery within its state of charge swing avoids the risk of degradation reactions during high power recharge, such as during rapid braking in a battery system used in HEVs. The risk of the occurrence of such degradation reactions increases as the state of charge increases. Therefore, limiting the maximum state of charge can extend the life of the battery. The stress and strain of deep charge and discharge can be detrimental to the battery resulting in reduced life leading to higher maintenance/operating costs of the HEV or plug-in HEV. Reducing the state-of-charge swing may also reduce the mechanical strain within either the positive or negative electroactive material, which improves battery longevity.
The quantities that can be easily measured during operation of most battery packs are: voltage, current, time, and temperature. Therefore, methods of estimating SOC usually take advantage of these inputs.
In some battery chemistries, e.g., lead-acid or lithium-ion batteries employing metal-oxide positive electrode materials, the voltage vs. capacity curve is not flat; that is, the voltage changes with respect to a change of capacity. These batteries therefore have a correlative relationship between voltage and capacity so that measurement of the battery voltage gives an approximation of the battery state-of-charge. Many SOC algorithms therefore use some voltage-based method to estimate SOC or to re-calibrate the SOC estimation. This, however, generally requires the battery chemistry to exhibit significant changes in open-circuit voltage with SOC, since that allows the SOC to be estimated based on the voltage change. A battery that exhibits a significant variation in impedance with SOC can also use the voltage change during a current pulse to determine SOC. Temperature is usually taken into account, as the impedance and open circuit voltage (OCV) may be a function of temperature.
The effectiveness of voltage-based methods is dependent upon there being some variation in voltage with capacity. Voltage or impedance-based algorithms are more difficult to implement in battery chemistries that demonstrate little or no change in impedance or OCV over a large range of SOC, and large errors in the SOC estimation may result. One example is a battery having a lithium iron phosphate positive electrode and a graphite negative electrode, because the voltage profiles of both the positive and negative electrodes are relatively flat so that only small changes in voltages are demonstrated over a wide capacity range.
Voltage-based methods are also subject to errors introduced by hysteresis. Hysteresis includes variations in the voltage vs. capacity or impedance characteristics of the battery that depend on its preceding charge or discharge history. For example, the voltage exhibited by the battery at a given fixed SOC may depend on whether the battery was last subjected to a charging or discharging event. The variation in voltage or impedance at given SOC can also depend on the rate of preceding charge or discharge events, the time elapsed reaching said SOC, and temperature. Lithium iron phosphate exhibits hysteresis during charge and discharge so that the recent history of the cell affects the voltage. Thus, the voltage measurement of a lithium iron phosphate cell does not provide a precise and accurate correlation to the battery capacity.
Coulomb counting is another commonly-used method of monitoring SOC, in which the current is integrated over time to determine how many amp-hours have been charged or discharged. The problem with coulomb counting, however, is that SOC estimation relies on accurate measurements at high and low current, as well as frequent data sampling. The hardware used for accurate SOC estimation is expensive, and coulomb counting alone can lead to errors which may accumulate over time. In order to reduce the accumulated errors, most SOC estimations also use methods of periodically re-calibrating the SOC estimate. Thus there is a need for simpler and more accurate methods of monitoring the SOC of batteries, and using less expensive equipment that can lower the overall cost of the system that is being powered.